With all the studies I've posted on my Web site recently about the benefits of eating chocolate, I ran across an interesting piece in the British Medical Journal (free text link below) that explains why the dark stuff can be fatal if your dog eats it.
The main components of chocolate that are toxic to dogs: Methylxanthine alkaloids theobromine and caffeine. Humans can easily digest and excrete these alkaloids in 2-3 hours, but dogs take a very long time -- 18 hours -- to absorb them, thus the problem. Often, symptoms can take as long as a day to appear, and no specific antidote exists.
Chock full of antioxidants that are beneficial to humans, unprocessed chocolate contains the purest, highest and most lethal concentrations of theobromine for dogs, while milk and white chocolate have a fraction of those alkaloids.
Another common sense variable to consider: The size of the dog -- small in this case is bad -- determines how damaging chocolate consumption will be.
While doing some research for this posting, I stumbled onto news about other foods that can be toxic if eaten by dogs and cats: Grapes and raisins.
British Medical Journal, Vol. 331, No. 633, September 17, 2005 Free Full Text Article